Formation mechanism of defects in Invar 36/MnCu functionally graded material fabricated by directed energy deposition

The fabrication of Invar/MnCu functionally graded material (FGM) through directed energy deposition (DED) can satisfy the demands for precision devices in aerospace, providing lightweight properties and integrating thermal stability and vibration damping capabilities. However, basic research on Invar/MnCu FGM is still lacking, hindering its potential applications. To address this gap, this study was conducted using mixed powders and consistent process parameters to print experiments for Invar/MnCu FGM and homogeneous samples. Phases, microstructures, compositions, and thermal expansion properties were thoroughly examined. Three types of defects were detected in the Invar/MnCu FGM sample: unmelted Invar 36 powders, cracks, and pores. The mechanism of unmelted powders was deeply discussed, attributing it to material properties influencing laser absorptivity, the required time for melting powder, and effects on solidus temperature. The mechanism of cracks was also discussed, attributing it to the γ-Fe dendritic structure causing low melting point metal to form an intergranular liquid film, harmful secondary phases mismatched with the terminal alloy, and obvious tensile stresses during the DED process. Additionally, an effective strategy was proposed to reduce defects in Invar/MnCu FGM. After optimization, the specimens exhibited excellent tensile properties, with a yield strength of 262 ± 5 MPa, an ultimate tensile strength of 316 ± 7 MPa, and an elongation of 3% ± 1%. This research provides valuable references and insights for subsequent work, offering robust support for better understanding and designing other FGM.